1. Field of the Invention
The invention relates to molecular beam epitaxial (MBE) processing and to means for MBE processing. More particularly the invention relates to the provision of beam forming material and beam formation and to a novel crucible by which the beam formation material is provided, and which creates the molecular beam.
2. Prior Art
In a molecular beam epitaxial system, a plurality of materials are disposed in furnaces to form effusion cells in proximity to a semiconductor wafer. In the MBE process an epitaxial layer is formed on the wafer, whose composition is controlled by the simultaneous operation of two to four effusion cells. The simultaneous operation produces an epitaxial layer having suitable properties for the formation of semiconductor devices and circuits. The process is conducted in a vacuum, with both the substrate and the material from which the molecular beam is formed being operated at elevated temperatures. The substrate is operated at an elevated temperature suitable for epitaxial growth, 600 degrees C. being suitable for gallium arsenide substrates.
The growth material is supplied by the effusion cells, for which the furnaces provide the necessary heat to achieve a desired level of effusion. Within the furnaces, it is conventional to employ inert crucibles, which contain the effusion material. In the case of the growth of gallium arsenide epitaxial layers separate crucibles are provided for gallium, which liquifies at relatively low temperature, for arsenic and for each other material used. Other materials such a silicon may be used to establish the "doping" levels, and thereby the conductivity of the resultant epitaxial layer.
In the physical arrangement, the effusion process entails growth materials which are both solid and liquid at the temperatures required to provide adequate effusion flux. The crucibles accordingly differ in design dependent on the operating position within the MBE system and on whether the material is liquid or solid during effusion.
Gallium, one of several materials that are important in epitaxial processing, is a liquid at effusion temperatures. Other Group III metals such as Indium, Gallium, or Aluminium, also require a crucible suitable for a liquified charge.
The criteria of a good crucible design come from the operational requirements. The crucible, given the limited spatial constraints of available MBE furnaces, should have a large melt capacity to reduce the frequency of recharging.
In the effusion process, the crucible should produce a beam which is uniform across its cross-section to support the formation of epitaxial layers of uniform thickness and constitution over the surface of the wafer.
In addition, since the processing requires the facility to make thin, reproducible layers, the crucible should permit accurate temporal control. In relation to reproducibility, the conventional means of initiation and terminating the effusion is by a mechanical shutter in proximity to the crucible. Operating the shutter effects a change in the temperature at the surface of the charge. This occurs because the shutter changes the radiative shielding of the charge in the crucible. The result is that upon opening the shutter, the initial effusion rate is excessive and eventually stabilizes at a lower value. Accordingly, an objective is a crucible design in which the melt temperatures--which control the rate of effusion--are insensitive to changes in the radiative shielding at the crucible orifice.
Group III melts the usually contained in pyrolitic boron nitride (PBN) crucibles of various shapes and sizes. The basic concepts of crucible design for achieving uniform deposition over 2 or 3 inch diameter wafers are well understood (see K. Ploog, "Molecular Beam Epitaxy of III-V Compounds", "Crystal Growth Properties and Applications", 1980, Vol. 3, Springler-Verlag; and P. E. Luscher and D. M. Collins, "Design Considerations for Molecular Beam Epitaxy Systems", "Progress in Crystal Growth and Characterization", 1979, Vol. 2, pp. 15-32, Pergamon Press Ltd.). Unfortunately for many of these cell configurations the melt temperature is affected by the radiation shielding provided by the beam shutter. A flux transient typically lasting one to three minutes occurs when the shutter is opened and the cell establishes a new equilibrium temperature.
Group III flux transients in MBE growth limit film reproducibility as well as being an inconvenience to the MBE user. Short term flux transients cause poor control over growth stoichiometry and uncertainty over initial growth rates. Control of growth rates during the first few minutes after shutter opening is important for the reproducible growth of such submicron structures as the high electron mobility transistor.
Present crucible designs provide either excellent uniformity and large flux transients or poor uniformity with small flux transients. Uniformity of molecular beam patterns depends on the source to substrate spacing and the angular flux distribution at the source. The best uniformity is obtained with a large source to substrate spacing and an emitting flux distribution at the cell orifice which is isotropic in the solid angle subtended by the substrate.
A known conically shaped crucible, shown in FIG. 2A, has these flux characteristics as well as a large cell orifice for high flux density. Flux uniformity at the source is attained by allowing a direct path to the entire substrate from each point on the melt surface. The atoms which escape the crucible without wall collisions are the main component of the flux. The cone angle and hence depth of the crucible is determined by the solid angle subtended by the cell orifice and substrate peripheries. The depth limits the cell volume and places the melt surface of a full crucible near the crucible orifice, where themelt temperature is sensitive to the shutter position.
Flux transients can also be reduced by the use of partially filled crucibles so that the influence of changes in the radiative shielding provided by the shutter are reduced. This approach is impractical as the reduced charge volume limits machine operation times between cell recharging. Schaff (Dr. W. Schaff Ph.D. Dissertation, Cornell University, 1984) has found that partially filled deep crucibles, shown in FIG. 2B, exhibit a smaller flux transient while retaining a large volume. This is because the melt temperatures deep within the furnace are less sensitive to changes in radiation shielding at the cell orifice. Deep crucibles tend to collimate the beam, however, which limits uniformity.
Accordingly, it is an object of the invention to provide an improved crucibiel for the production of molecular beams in a molecular beam epitaxial (MBE) system.
It is still another object of the invention to provide a crucible for the production of molecular beams in an MBE system having improved melt capacity while providing a uniform beam and low flux transient behavior.
It is an additional object of the present invention to provide an improved method of beam formation in an MBE system.
These and other objects of the invention are achieved in a novel crucible for use in a molecular beam expitaxial (MBE) system.
The crucible comprises an outer member of refractory material of relatively large volume for supporting a relatively large quantity of melt at a substantial depth, and an inner member of refractory material, set within the outer member, of a generally conical cross-section, having a large opening oriented toward a substrate supported for rotation within the MBE equipment and a small opening oriented toward the melt.
The outer member, given the limited internal dimensions of the furnaces of an MBE system, permits the melt to be disposed at an increased distance from the substrate and the inner member provides an additional radiation barrier to reduce the thermal transient which occurs when the shutter is operated. The dimensions of the inner member are selected to expose a substantially constant melt area to all points across the substrate at a given level of melt, and a slightly increasing exposed melt area as the level of melt falls in the outer member. These provisions lead to greater flux uniformity over the substrate surface and over processing time.
In accordance with another aspect of the invention, the area of melt exposed to the substrate via the inner member is from one-third to one-fifth the total melt area to reduce heat loss and regulate the exposed melt area while avoiding formation of a vapor equilibrium over the melt.
The inner conical member is designed such that the solid angle is substantially equal to the solid angle subtended by the substrate from a circular locus of point sources on the melt in positions adjacent the perimeter of the small opening of the conical member. This minimizes molecular or atomic trajectories completed to the substrate if a prior deflection by the conical member has occurred.